Another macro that can be used for quite a lot of items:

<% story.url %>
<% comment.url %>
<% site.url %>
<% image.url %>
<% file.url %>

The result of these macros is - surprise! - the url to the story, comment, site, image or file.

Additionally, you can use the "action" parameter to append a specific action name after the url:

<% site.url action="referrers" %>